Water softeners and the like use ion exchange resins, such as zeolite resins, to remove polyvalent ions from aqueous streams by exchanging them with monovalent cations such as sodium. When there are few sodium ions left on such resins, it becomes necessary to regenerate the ion exchange resins by treatment with an aqueous brine containing sodium chloride or the like, e.g. potassium chloride. As a part of this regeneration, the polyvalent cations are solubilized and replaced on the resin with sodium ions. The resultant aqueous solution contains some sodium chloride plus the solubilized polyvalent ions and is generally referred to as "spent brine". Although heretofore it has usually been acceptable to simply discharge such spent brine into a municipal sewer system, environmental restraints are being imposed throughout many parts of the United States, and the need to conserve water has also dictated that efforts be made to reclaim such spent brine.
In May of 1992, Larry Lien of Membrane Development Specialists, Inc. was asked to speak at the semiannual meeting of the Pacific Water Quality Association in Santa Barbara, Calif. At that meeting, the concept of reclaiming spent brine used to regenerate water-softening resins was recommended, and it was suggested a semipermeable membrane separation treatment, specifically one using spirally-wound elements or cartridges made of the DS-5 nanofiltration membrane being marketed by Desalination Systems, Inc., might be advantageously employed. The adoption of such spent brine reclamation systems was advocated so as to reduce the cost of sodium chloride and reduce sewer charges, as well as because it would be an environmentally sound practice.
In July of 1992, an article was published in Water Technology entitled "Brine Recovery" that was authored by Howard Connor which likewise advocated the recovery of spent brine. In September of 1992, Larry Lien published an article entitled, "Nanofiltration: Trend of the Future?" in Water Conditioning and Purification describing uses of such nanofiltration elements for a variety of applications. This article was followed with a presentation at the Annual Convention of the Water Quality Association in San Antonio, Tex. (March 1993).
In January of 1993, a United States patent application was filed, which issued later that year as U.S. Pat. No. 5,254,257. It is directed specifically to the concept of purifying spent brine from the regeneration of ion exchange resins. The patent discloses the use of DS-5 nanofiltration membrane cartridges for spent brine reclamation by first acidifying the spent brine with hydrochloric acid to a pH of about 1.5 to 2.4 and subsequently treating the purified or reclaimed brine with sodium hydroxide to raise the pH.
Sodium chloride has long been used as a feed material to electrolytic cells where electrolysis produces chlorine and sodium hydroxide, and processes have been developed for removing polyvalent cations, such as calcium, magnesium, iron, chromium, barium, strontium, nickel, molybdenum and the like from such sodium chloride brines prior to electrolysis. Such processes have generally involved the use of chemicals that cause precipitation of these polyvalent ions. U.S. Pat. Nos. 4,277,447 and 4,336,232 are generally representative of such processes for purifying sodium chloride or the like brines by the removal of such polyvalent cations. U.S. Pat. No. 5,587,083 discloses the removal of divalent ions from an aqueous chlorate feed liquor using nanofiltration membranes.
The demands on water resources in the United States have continued to increase, and environmental regulations with respect to sewer discharges continue to grow even more stringent. As a result, efforts have continued for the purpose of developing even more efficient reclamation processes for spent brine.